It is often desirable to control the temperature of a sample that is being studied in a scanning probe microscope (SPM) such as a scanning tunneling microscope (STM) or an atomic force microscope (AFM). This is because many aspects of surface structure and chemistry are sensitive to temperature, so variable temperature operation significantly increases the utility of the scanning probe microscope.
Scanning probe microscopes have been constructed which operate in a cryogenic fluid or inside a high vacuum chamber. In each case it is relatively straightforward to control the temperature of the sample, and/or the microscope as well. However, in the case of microscopes designed to operate in ambient air or some gas at or near ambient pressure, it is more difficult to design a heated or cooled sample stage. The reason is that convection caused by hot or cold gasses and temperature gradients across the microscope (the body of which is assumed to be at ambient temperature) causes mechanical instabilities which degrade the resolution of the microscope.
More specifically, current variable temperature designs suffer both from thermal expansion and stage heating issues, which impact the ability of users to perform temperature-dependent measurements in a scanning probe microscope. When thermal expansion or contraction occurs, the piezoelectric actuators must compensate in an opposite direction, often using up their dynamic range. Lateral drift of the scanned region also occurs and is detrimental to experiments. Furthermore, many scanning probe microscopes employ a sample scanning configuration, in which the sample is mounted on a stage that is directly coupled to the piezoelectric actuators. Heating the sample in this configuration can also overheat the piezoelectric materials, leading to inaccuracy or failure of the scanner. Accordingly, conventional variable temperature experiments are either limited in temperature or must be performed on tip/probe scanning microscopes. Nonetheless, thermal expansion/contraction by the heater stage affects all types of scanning probe microscopes.
Although various strategies have been described in the prior art, a need remains for an effective approach for reducing, or ideally eliminating, motion of a sample caused by thermal expansion or contraction of the stage or other member supporting the sample.